1. Field of the Invention
The present invention concerns soft switching power converters, in particular the power converters described in the article "High Power Transistor Inverters--Potential for Single Device Operation at 1000 A and 800 V", by G. A. Fisher published in the minutes of the "16th Universities Power Engineering Conference", Sheffield, Great Britain, in the article "Resonant Snubbers with Auxiliary Switches", by W. McMurray, published in IEE Transactions on Industry Applications, Vol.29, No.2, March/April 1993, and in patent document WO 92/05625.
2. Description of the Prior Art
FIG. 1 shows a converter leg of the type described in the publications mentioned. This converter leg is adapted to power a load L, shown as essentially inductive in nature, for example a motor, from a direct current voltage supply provided between a positive rail VR+ and a negative rail VR-. It essentially comprises:
two main switches S1 and S2 respectively connecting an output S leading to the load L either to the rail VR+ or to the rail VR-, PA1 two freewheel diodes D1 and D2 for maintaining the current in the load L by respectively coupling the output S either to the positive rail VR+ or to the negative rail VR-, PA1 two snubbers C1 and C2 between the output S and the positive rail VR+ and the negative rail VR-, respectively, PA1 an auxiliary circuit CA including an auxiliary inductor LA in series with two antiparallel unidirectional conduction auxiliary switches T1 and T2, for example thyristors, or functionally equivalent circuits, connected between the output S and the mid-point M of a capacitive voltage divider DC; the latter in turn comprises two divider capacitors CV1 and CV2 connected in series between the positive and negative rails VR+ and VR- and supplying at this mid-point M a nominal median voltage relative to those of the positive and negative rails VR+ and VR-.
In this circuit, the components S1, D3, C1, T1, CV1 are respectively paired with components S2, D2, C2, T2, CV2, i.e. they have the same electrical characteristics at all times. A control circuit, not shown, measures voltages and currents at various points of the converter including this leg and commands switching of the main switches S1 and S2 and the auxiliary switches T1 and T2 in accordance with an appropriate program, by action at control electrodes that are merely sketched in in the figure, at ec1, for example, in the manner now to be described.
To illustrate the operation of this circuit in the case of a DC--DC converter, the leg from FIG. 1 operating as a chopper, the starting state is one in which no switch is turned on and a current IL is maintained in the load L by the freewheel diode D2 which is turned on at this time. Accordingly, ignoring the threshold voltage of the diode D2, the output S is at the same potential as the rail VR-, for example 0 volt. The capacitors CV1 and CV2, which have the same capacitance, are charged to the voltage +V between the rails VR+ and VR-, with the result that the mid-point M is at the median voltage +V/2.
Commutation of the circuit begins with a first step in which the auxiliary switch T1 is turned on by the control circuit. The auxiliary inductor LA is energized between +V/2 (ignoring the threshold of V1) and 0 V (ignoring the threshold of D2); the current IA through it increases linearly. This current is subtracted from the current IL in the diode D2.
When IA=IL the diode D2 turns off. The output S is no longer connected to the rail VR-. The auxiliary inductor LA is then in series with the capacitors C1 and C2, which as far as it is concerned are in parallel with each other. An oscillatory current arises between the inductor and the capacitors. In the first half-wave of this oscillation the mid-point of the snubbers C1 and C2, i.e. the output S, goes from 0 to +V volts.
The switch S1 is then turned on, with no voltage across it, and takes the current IL. The current IA flowing through the inductor LA, now connected between the mid-point M, at the potential +V/2, and the output S, maintained at the potential +V via the switch T1, decreases linearly. When it falls to zero the auxiliary switch T1 turns off. For this reason a thyristor type switch is employed, which has the property of turning off when the current passing through it falls to zero, or any equivalent combination of components, a transistor or an IGBT in series with a diode, for example.
Commutation in the other direction, to return to the initial situation, is accomplished in a similar fashion, using the auxiliary circuit CA, the auxiliary switch T2 of which is turned on, as the auxiliary switch T1 was before. Moreover, the switch S1 is then also turned off. The voltage at the output S then goes from +V to 0 volts in one oscillation at the end of which the auxiliary switch T2 turns off, the diode D2 then turning on due to the effect of the current IL.
Other modes of use of the converter leg described can be envisaged, in particular that of FIG. 2 in which the load is connected between two converter legs identical to that of FIG. 1, the components of the second leg carrying the same reference numbers as the first with the addition of a "'". Additionally, and as a variant, the inductors LA and LA' are connected between the auxiliary switches and the point M, instead of being connected between these auxiliary switches and the point S; this does not change anything with regard to the operation as described here. Other modes of operation can be applied in a two-leg converter such as that shown in FIG. 2, in particular inverter operation. The operation of each of the two legs is tied to what has just been described. In chopper mode operation, the operation of the lefthand leg alternately involves the diode D2 and switch T1, while, synchronously, the operation of the righthand leg involves the diode D1' and the switch T2'. In inverter operation, after a positive half-cycle, or several parts of a positive half-cycle, in accordance with what has just been stated, a negative half-cycle will involve, on one side, the diode D1 and then the switch T2 and, on the other side, the diode D2' and then the switch T1.
In these various situations, the voltage is switched at the output S of a converter leg without losses if the voltage at the output S goes from 0 to +V volts, that is to say if the voltage across the auxiliary circuit CA is reversed, that is to say if the voltage at the point M is equal to +V/2. For this, the capacitances of the capacitors CV1 and CV2 of the voltage divider DC must be sufficiently high and the symmetry of the components involved in the conduction phases of the auxiliary circuit must be quasi-perfect. Experience shows that these conditions are difficult to meet and this constitutes a problem.
In the case of the symmetrical converter from FIG. 2, it will be noted that the two legs share the same voltage divider DC. The currents required of this voltage divider are in opposition and in principle equal. However, the problem referred to above nevertheless arises in practice.
The skilled person will easily envisage an application of the converter just described in the context of a three-phase circuit. The same problem is still present.
The invention supplies an effective, low-cost solution to this problem.